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SUCCESSIONS  OF  VEGETATION  IN 
BOULDER  PARK,  COLORADO 


A DISSERTATION 
SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 


DEPARTMENT  OF  BOTANY 


BY 

WILFRED  WILLIAM  ROBBINS 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 


Reprinted  from 

The  Botanical  Gazette,  Vol.  LXV,  No.  6 
June,  1918 


Ube  xnntvetsttg  of  Chicago 


SUCCESSIONS  OF  VEGETATION  IN 
BOULDER  PARK,  COLORADO 


A DISSERTATION 
SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 


DEPARTMENT  OF  BOTANY 


BY 

WILFRED  WILLIAM  ROBBINS 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 

Reprinted  from 

The  Botanical  Gazette,  Vol.  LXV,  No.  6 
June,  1918 


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Digitized  by  the  Internet  Archive 
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University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/successionsofvegOOrobb 


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VOLUME  LXV  NUMBER  6 

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9 

THE 

Botanical  Gazette 


JUNE  igi8 


SUCCESSIONS  OF  VEGETATION  IN  BOULDER  PARK, 

COLORADO 

CONTRIBUTIONS  FROM  THE  HULL  BOTANICAL  LABORATORY  238 

W.  W.  Robbins 
(with  FOURTEEN  figures) 

Introduction 


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This  paper  is  concerned  chiefly  with  the  plant  successions  of 
the  flood  plains,  lakes,  and  ponds  of  a mountain  park.  They 
culminate  in  a temporary  meadow  climax.  Consideration  is  also 
given  to  the  succession  which  begins  on  the  xerophytic  glacial 
gravels  and  passes  through  the  characteristic  and  long  persistent 
“dry  grassland”  stage  also  to  a temporary  meadow  climax.  There 
is  also  presented  the  interesting  problem  of  the  genetic  relation  of 
the  meadow  to  the  forests  of  aspen,  lodgepole  pine,  and  Engelmann 
spruce-balsam  fir  which  border  the  open  park.  It  should  be  stated 
that  Boulder  Park  is  typical,  in  its  physiographic  and  vegetative 
development,  of  hundreds  of  such  areas  in  the  Rocky  Mountain 
region. 

Boulder  Park  is  located  in  Gilpin  County,  Colorado,  about 
34  miles,  in  a straight  line,  west  of  Denver.  The  Divide  of  the 
main  range  of  the  Rocky  Mountains  is  about  6 miles  west;  the 
Great  Plains  are  about  18  miles  east.  Tolland,  a small  town  near 
the  middle  of  the  Park,  has  an  altitude  of  8889  feet.  The  area 
falls  within  the  montane  zone  (10). 

493 


494 


BOTANICAL  GAZETTE 


[JUNE 


Boulder  Park  was  selected  as  the  site  for  the  University  of 
Colorado  Mountain  Laboratory,  the  first  session  being  held  in  the 
summer  of  1909.  Several  papers  (11,  12,  19)  have  been  issued 
setting  forth  the  facilities  for  field  study  at  the  Laboratory,  but 
also  presenting  certain  botanical  features  of  the  neighboring  vegeta- 
tion. Other  papers  dealing  exclusively  with  the  plant  life  of  the 
Park  and  adjacent  territory  are  referred  to  in  this  paper. 

Topography  and  physiographic  history 

General. — The  term  “park”  as  used  throughout  the  Rocky 
Mountain  region  refers  to  an  open,  flat,  usually  grassy  area  in  the 
mountains.  Such  areas  may  be  large,  including  100  or  more  square 
miles;  or  small,  containing  only  a portion  of  a square  mile,  and 
often  possess  a scattered  growth  of  trees.  Boulder  Park  is  the 
broadened  valley  of  South  Boulder  Creek.  It  is  generally  level,  and 
through  it  flows  the  stream  which  is  slowly  working  its  way  back 
and  forth  and  producing  well  defined  flood  plains.  The  level 
portion  of  the  park  proper  is  bordered  by  steep  slopes,  the  crests 
of  which  are  500-1000  ft.  above  the  valley  floor.  The  slopes  have 
been  burned  over  in  large  part,  and  exhibit  various  stages  of  the 
“burn  succession,”  the  most  obvious  of  which  are  lodgepole  pine 
and  aspen.  The  climatic  climax  forest  of  Engelmann  spruce-  balsam 
fir  is  found  in  places.  Some  typical  talus  slopes  occur.  The 
country  granitic  rock  is  exposed  in  rugged  outline  in  places,  and 
may  be  observed  in  different  stages  of  disintegration  and  decom- 
position, and  in  various  stages  of  vegetative  development.  The 
building  of  the  Moffat  Railroad  has  created  deep  rock  and  gravel 
cuts,  gravel  and  rock  heaps  and  slides,  often  resembling  talus  and 
natural  slides;  likewise,  the  building  of  wagon  roads  has  made  many 
new  areas  whereupon  secondary  succession  may  be  observed.  The 
general  topographic  features  of  the  Park  may  be  seen  by  referring 
to  figs.  1-3. 

In  Pleistocene  times  this  area  and  many  others  in  the  mountains 
of  Colorado  were  glaciated.  The  glaciers  had  their  heads  above 
timber  line  and  moved  down  the  valleys,  leaving  evidence  of  their 
action.  The  ice  which  was  largely  instrumental  in  shaping  and 
modifying  the  topography  of  Boulder  Park  came  from  two  sources, 


Fig.  i. — Map  of  Boulder  Park  (surveyed*by  G.  S.  Dodds,  \V.  L.  Brosius,  and  W.  W.  Robbins) 


ROBBINS— BOULDER  PARK 


495 


1918] 

South  Boulder  Canyon  and  Mammoth  Gulch.  The  two  bodies 
of  ice  no  doubt  met  at  the  upper  end  of  the  Park  and  filled  it  to  a 
depth  of  400-550  ft.,  as  is  evidenced  by  the  elevation  of  perched 
boulders  on  both  slopes  bordering  the  Park.  Below  Tolland  is  an 
area  of  hummocky  topography  typically  morainal.  The  hum- 
mocks and  the  depressions  between  are  strewn  with  rounded 
boulders,  so  much  weathered  that  all  traces  of  glacial  scratches  are 
obliterated.  Post-glacial  stream  action  has  washed  away  great 
quantities  of  this  terminal  moraine.  Lateral  moraines  join  with 
the  terminal  and  extend  up  the  valley  on  the  sides  of  the  ridges. 
There  is  also  a very  wide  and  deep  morainal  deposit  at  the  mouth 
of  Mammoth  Gulch.  A comparatively  small  amount  of  it  has  been 
carried  away.  At  the  entrance  of  South  Boulder  Canyon,  however, 


Fig.  2. — Profile  of  Boulder  Park  along  line  extending  from  A to  B in  fig.  1 


but  small  remnants  of  moraine  remain.  This  is  undoubtedly  due 
to  the  fact  that  South  Boulder  is  a much  larger  stream  than  Mam- 
moth, and  since  the  retreat  of  the  glacier  it  has  carried  away  almost 
all  of  its  moraine. 

It  has  just  been  indicated  that  there  are  two  large  distinct 
deposits  of  morainal  material  in  the  Park:  one  below  Tolland,  the 
other  at  the  mouth  of  Mammoth  Gulch.  They  probably  represent 
the  terminals  of  two  distinct  glacial  bodies  of  ice  belonging  to  differ- 
ent periods  of  glaciation.  In  all  instances  where  investigations 
(1,  4,  7,  8,  9,  23,  26)  of  the  epochs  of  glaciation  have  been  made 
in  the  western  mountains,  there  have  been  two  distinct  epochs, 
and  furthermore,  in  each  case  the  earlier  glacier  extended  farther 
than  the  later. 

Origin  of  terraces. — The  foregoing  has  been  given  in  order 
to  make  clear  the  origin  of  the  ponds  and  terraces  which  are  such 
prominent  features  of  the  Park’s  topography.  Figs.  1 and  2 show 


496 


BOTANICAL  GAZETTE 


[JUNE 


that  there  are  3 main  levels  bordering  the  stream:  (1)  Low 
terraces,  into  which  the  present  stream  is  now  cutting.  During 
high  waters  a portion  of  this  may  be  inundated.  Gravel  is  overlaid 
with  a deposit  of  peat,  ranging  in  depth  from  an  inch  or  so  to  3 or 
4 ft.  This  level  is  covered  with  a willow  thicket  association  in 
which  the  dominant  forms  are  Salix  chlorophylla  Anders,  and 


Fig.  3. — View  of  Boulder  Park  looking  west:  James  Peak  at  left  and  Continental 
Divide  in  background ; glimpses  of  stream  may  be  seen  flowing  through  willow  thicket 
association;  note  lighter  colored  patches  of  dry  grassland;  slopes  are  clothed  with 
aspen,  lodgepole  pine,  and  Engelmann  spruce-subalpine  fir. 

S.  padophylla  Rydb.  (2)  Middle  terraces,  meander  terraces  much 
older  than  the  preceding.  On  the  middle  level  the  gravel  is  not 
overlaid  with  peat,  except  in  a few  places.  The  soil  is  a sandy  and 
gravelly  loam  and  4-8  inches  deep.  The  characteristic  vegetation 
is  meadow  scrub  in  which  Salix  glaucops  Anders.,  Betula  glandulosa 
Sarg.,  Dasiophora  jruticosa  (L.)  Rydb.,  and  mesophytic  herbs  are 
dominant.  (3)  High  terraces,  which  are  composed  of  glacial  gravels 
which  have  been  worked  over  by  the  stream  which  issued  from  the 


ROBBINS— BOULDER  PARK 


497 


1918] 


end  of  the  retreating  glacier.  Typically,  there  is  little  accumula- 
tion of  humus  upon  them.  The  vegetation  is  a “dry  grassland.” 

The  terminal  moraine  below  Tolland  undoubtedly  acted  as  a 
dam  to  the  stream  coming  from  the  glacial  front,  and  for  a long 
time  the  Park  was  the  site  of  a lake.  Subsequent  to  the  period  of 
deposition,  the  stream  was  lightened  of  its  load  and  immediately 
began  to  cut  into  the  terminal  dam,  which  being  of  easily  eroded 
material  was  quickly  cut  through.  This  resulted  in  a rapid 
drainage  of  the  lake  and  the  formation  of  a high  terrace  on  each 
side  of  the  stream  issuing  from  the  glacial  front.  The  middle 
terraces  are  of  stream  origin. 

Origin  of  lakes  and  ponds. — With  the  exception  of  Park  and 
Filled  Lakes,  all  the  natural  ponds  in  the  Park  are  of  oxbow  origin. 
Park  and  Filled  Lakes  are  the  largest  and  deepest.  Their  depth 
alone  shows  that  they  are  not  of  oxbow  origin.  In  the  center  of 
Filled  Lake  the  peat  is  over  10  ft.  deep.  At  no  other  point  in  the 
Park  is  there  such  a deep  peat  deposit.  The  relation  these  two 
lakes  bear  to  the  higher  level  shows  that  they  were  not  formed 
during  the  deposition  of  the  material  composing  this  level  On  the 
retreat  of  the  first  glacier  two  large  ice  cores  were  left  on  the  valley 
floor.  Hence,  when  the  wash  from  the  later  terminal  was  brought 
down  the  valley  it  was  deposited  about  the  edges  of  these  debris 
covered  bodies  of  ice.  The  ice  melted  later,  leaving  the  two  lakes, 
Park  and  Filled. 

Climatic  factors 

There  are  no  extended  climatic  records  for  Boulder  Park. 
Robbins  (22)  has  shown  the  following  average  temperature  and 
precipitation  relations  to  exist  in  the  “lodgepole  pine  forest  zone” 
of  Colorado,  and  from  these  a notion  may  be  gained  of  climatic 
conditions  in  Boulder  Park. 

Mean  annual  temperature 34 . 90  F. 

Mean  winter  temperature 1 8 . 40 

Mean  spring  temperature 33  -4° 

Mean  summer  temperature 53  -6° 

Mean  fall  temperature 38.4° 

Average  length  of  frostless  season 67  days 

Average  date  of  last  spring  frost June  20 

Average  date  of  first  fall  frost September  9 

Absolute  annual  range  of  temperature 104  days 

Mean  annual  precipitation 25.53  inches 

Mean  annual  snowfall 112.9  inches 


498 


BOTANICAL  GAZETTE 


[JUNE 


During  a portion  of  the  growing  seasons  of  1909  and  1913, 
thermographs  were  run  by  members  of  the  staff  of  the  Mountain 
Laboratory.  They  show  that  the  daily  range  of  temperature  may 
run  high.  This  is  particularly  the  case  during  clear  weather.  The 
daily  minimum  temperature  is  usually  reached  between  5:00  and 
6:00  a.m.,  the  daily  maximum  between  1:00  and  2:00  P.M.  In 
1909  the  latest  freezing  temperature  was  June  22;  in  1913  the 
temperature  sometimes  went  down  to  freezing  or  below  throughout 
July,  and  on  August  1 of  that  year  the  minimum  was  320  F.  After- 
noon showers  of  short  duration  during  June,  July,  and  August  are 
common.  Prolonged  rains  are  infrequent.  Although  there  is 
considerable  snow,  its  accumulation  on  the  high  terraces,  particu- 
larly, is  largely  prevented  by  their  exposure  to  the  sweep  of  winds 
from  the  west.  There  are  large  drifts  of  snow,  however,  in  pro- 
tected situations. 

Ramaley  and  Mitchell  (18)  in  1909  determined  the  relative 
humidity  at  a number  of  Park  stations.  It  varied  on  July  8 from 
39  per  cent  on  the  north  exposure  of  a railroad  cut  to  65  per  cent 
in  the  lodgepole  pine  forest;  on  July  12,  from  39  per  cent  in  the 
railroad  cut  to  71  per  cent  in  the  forest. 

Successions 

FLOOD  PLAIN  SUCCESSION 

Boulder  Creek  is  a meandering  stream  with  considerable  cutting 
power.  Along  its  course  in  the  Park  one  may  find  shores  of  erosion, 
of  deposition,  and  numerous  oxbows  in  all  physiographic  and  vegeta- 
tive stages,  and  also  well  defined  stream  terraces.  Hence  there  is 
an  unexcelled  opportunity  here,  as  may  be  judged  somewhat  from 
a reference  to  fig.  1,  to  study  succession  on  a mountain  flood  plain. 

Two  types  of  embryonic  flood  plains  occur  along  Boulder  Creek 
in  its  course  through  the  Park:  (1)  those  composed  of  well  rounded 
boulders  (cobblestones),  averaging  2-6  inches  in  diameter,  with  a 
slight  admixture  of  coarse  gravel  (fig.  4) ; and  (2)  those  made  of 
sand  and  silt  (fig.  5).  The  former  are  initially  xerophytic,  the 
latter  hydrophytic. 

Shores  of  the  cobblestone  type  may  be  partly  under  water  during 
the  spring  or  in  wet  seasons,  but  are  usually  bare  in  summer  and 


1918] 


ROBBINS— BOULDER  PARK 


499 


during  dry  years.  The  temperature  extremes  are  great.  Further- 
more, the  occurrence  of  flood-waters  postpones  the  invasion  of 
pioneers.  The  freshly  exposed  stones  and  gravel  possess  no  vegeta- 
tion. Algae  which  may  have  been  clinging  to  rocks  while  sub- 
merged are  killed  on  exposure  to  the  sun.  There  is  no  lichen 
stage  on  the  rock  surfaces.  The  first  plants  gain  a foothold  in 


Fig.  4. — Along  South  Boulder  Creek:  flood  plain  of  cobblestones  invaded  by 
highly  mixed  plant  community;  note  zone  of  Car  ex  variabilis  bordered  outwardly  by 
willow  thicket. 

the  meager  accumulation  of  fine  sediment  between  the  stones. 
Agrostis  hiemalis  (Walt.)  B.S.P.  and  moss  species  usually  initiate 
the  succession.  The  individual  grass  plants  form  an  interlacing, 
dense  mass  of  fibrous  roots  which  collect  and  hold  sand  and  silt. 
These  initial  plants  are  followed  by  a highly  mixed  community 
composed  of  migrants  from  the  sedge  moor,  willow  thicket,  meadow, 
and  even  dry  grassland;  in  fact,  there  is  no  new  habitat  in  the 
Park,  except  it  is  the  roadside,  where  there  is  such  a great  number 


5°° 


BOTANICAL  GAZETTE 


[JUNE 


of  different  species.  The  principal  invaders  are  Deschampsia 
caespitosa  (L.)  Beauv.,  Phleum  alpinum  L.,  Poa  alpina  L.,  Sporo- 
bolus  brevifolius  (Nutt.)  Scribn.,  Car  ex  variabilis  Bailey,  Salix 
chlorophylla  Anders.,  S.  padophylla  Rydb.,  Sedum  rhodanthum 
Gray,  and  Dodecatheon  radicatum  Greene.  The  occurrence  here  of 
a depauperate  form  of  Erigeron  eximius  Greene  is  interesting,  as 


Fig.  5. — Invasion  of  flood  plain  of  sand  and  silt:  note  openness  of  vegetation, 
advancing  clumps  of  Agrostis  hiemalis  and  Alopecurus  fulvus,  and  society  of  young 
Salix  chlorophylla  and  S.  padophylla  shrubs. 


is  also  the  presence  of  such  species  as  Festuca  ingrata  (Hack.) 
Rydb.,  Rumex  acetosella  L.,  Arenaria  Fendleri  Gray,  Sedum  steno- 
petalum  Pursh,  and  Dasiophora  fruticosa.  In  spite  of  the  large 
number  of  species,  the  community  is  open.  As  vegetative  develop- 
ment proceeds  there  is  a reduction  in  the  number  of  species  and 
an  increase  in  the  number  of  individuals  of  the  successful  species, 
and  increasing  mesophytism  of  the  habitat. 


ROBBINS— BOULDER  PARK 


1918] 


501 


As  is  quite  commonly  the  rule  on  flood  plains,  the  first  woody 
plants  to  gain  a footing  are  Salix  species.  In  this  case  the  invaders 
are  S.  chlorophylla  and  5.  padophylla  (figs.  4,  5).  After  a few  years 
a willow  thicket  is  formed;  at  the  present  time  extensive  willow 
thickets  prevail  along  the  whole  course  of  the  stream  on  ground 
not  far  removed  from  the  water  level.  The  willow  thicket  has  a 
peat  deposit  from  a few  inches  to  over  3 ft.  deep.  As  has  been 
indicated,  the  water  level  is  near  the  surface  at  all  times,  and  the 
association  may  undergo  flooding  in  the  early  season. 

In  the  series  of  successions  starting  with  gravelly  and  stony 
stream  banks,  willow  thicket  is  replaced  by  a meadow  scrub.  The 
presence  of  tall  willows  about  a terrace  lake  in  the  Park  has  come 
to  be  looked  upon  as  evidence  of  its  oxbow  origin.  The  dying  out 
of  tall  willows  in  the  drier  portions  of  the  willow  thicket,  the  quite 
common  presence  of  relicts  of  willow  thicket  throughout  the 
meadow  scrub,  and  the  occurrence  of  small  patches  of  meadow 
scrub  throughout  the  willow  thicket,  are  all  evidences  that  willow 
thicket  is  being  succeeded  by  meadow  scrub.  Such  relicts  are 
usually  represented  by  a few  tall  Salix  padophylla  and  S.  chlorophylla 
shrubs,  and  in  almost  all  instances  such  individuals  possess  many 
dead  branches. 

Meadow  scrub  attains  its  typical  structure  on  the  middle 
stream  terrace.  The  characteristic  shrubs  are  Salix  glaucops  and 
Dasiophora  fruticosa,  both  of  which  are  low  forms  as  compared 
with  those  shrubs  dominating  the  willow  thicket.  The  herbs  are 
those  found  in  the  herbaceous  meadow  of  the  Park.  Meadow 
scrub  commonly  has  a striking  hummocky  character.  This  is  due 
to  herbs  building  up  about  the  shrubs.  In  places  Dasiophora 
dominates  the  association.  This  shrub  stands  about  18  inches 
high,  and  the  individuals  usually  2 or  3 ft.  apart.  It  has  a con- 
siderable habitat  range.  In  the  progressive  drying  of  the  meadow 
scrub,  it  lags  behind  as  a relict.  It  is,  on  the  other  hand,  a common 
invader  of  the  sedge  moor. 

The  fact  that  meadow  scrub  on  the  middle  terrace  is  laid  on 
gravel  indicates  that  the  stream  must  have  moved  laterally  rapidly 
on  that  level,  thus  giving  little  opportunity  for  the  development  of 
peat.  At  the  present  time  the  stream  is  swinging  back  and  forth 


502 


BOTANICAL  GAZETTE 


[JUNE 


across  the  valley  at  a comparatively  slow  rate,  as  is  witnessed  by 
the  formation  of  peat  on  the  recent  levels. 

Occasionally  sedge  moor  may  precede  willow  thicket  on  stream 
banks,  and  it  is  not  at  all  uncommon  to  find  stream  banks  of  shingle 
remain  xerophytic  for  a long  period.  The  small  isolated  dry  grass- 
land patches  throughout  the  willow  thicket  association  are  undoubt- 
edly of  this  type. 

Where  the  meander  approaches  its  maximum  curvature,  the 
force  of  the  stream  on  the  inside  of  the  curve  is  so  slight  that  fine 
material  is  freely  deposited.  A good  illustration  of  this  is  to  be 
seen  in  oxbow  20  (fig.  5).  The  main  current  flows  through  the 
cut-off  channel.  A small  portion  of  the  stream  with  only  slight 
carrying  power  still  flows  through  the  oxbow.  It  has  built  up 
a sandy  and  muddy  stream  flat.  Such  a habitat  as  this  has  a 
varied  vegetative  history.  Usually,  an  association  of  Eleocharis- 
Ranunculus  is  the  first  to  become  established.  This  is  the  char- 
acteristic amphibious  community  of  the  Park.  The  principal 
species  are  Eleo  char  is  acicularis  (L.)  R.  and  S.,  E.  palustris  (L.) 
R.  and  S.,  Ranunculus  replans  L.,  R.  natans  L.,  Allocarya  scopu- 
lorum  Greene,  and  Alopecurus  fulvus  (L.)  R.  and  S.  Eleo  char  is 
acicularis  builds  a dense  turf  or  mat.  Allocarya  scopulorum  may 
also  grow  so  thickly  as  to  form  a rather  close  growth  over  the  soil 
surface.  Alopecurus  fulvus  is  a constant  principal  species  of  the 
community.  Eleo  char  is  acicularis  often  grows  into  several  inches 
of  water;  such  plants  are  sterile.  However,  by  a slight  lowering 
of  the  water  level,  the  plants  spread  rapidly  both  by  the  under- 
ground parts  and  by  seed,  and  in  one  season  may  make  good  head- 
way toward  reclamation  of  the  mud  flat  exposed.  Eleocharis 
palustris  (fig.  6)  finds  its  best  expression  in  some  of  the  oxbow  lakes, 
especially  those  that  have  a flat,  stony,  or  gravelly  bottom  and 
possess  water  only  a part  of  the  year.  In  oxbow  lakes  8 and  9,  for 
example,  almost  the  entire  area  over  which  water  stands,  for  a time 
at  least,  is  covered  with  Eleocharis  palustris  and  Ranunculus 
reptans.  Rare  associates  are  Glyceria  borealis  (Nash)  A.  Nels.  and 
G.  grandis  Wats. 

The  Eleocharis-Ranunculus  association  is  followed  usually  by 
sedge  moor,  in  which  Carex  variabilis  is  the  predominant  species, 


ROBBINS— BOULDER  PARK 


503 


i9i8[ 

and  this  by  a willow  thicket  of  Salix  chlorophylla  and  S.  padophylla, 
or  in  certain  instances  the  mud  flat  along  streams  may  be  invaded 
directly  by  Salix  species,  and  still  in  other  cases,  especially  where 
the  soil  is  sandy  rather  than  muddy,  Agrostis  is  initial  and  is  fol- 
lowed by  a mixed  community  similar  to  that  on  more  gravelly 


Fig.  6. — Along  shore  of  oxbow  lake  9:  at  shore  edge  there  is  an  almost  pure 
association  of  Carex  utriculata;  bordered  on  water  side  by  Eleocharis  palustris- 
Ranunculus  association;  tall  willows  on  farther  side  are  relicts  of  willow  thicket  stage. 

stream  banks.  This  is  replaced  by  willow  thicket,  which  in  turn 
gives  way  to  a mesophytic  grassland  or  meadow  scrub. 

Oxbow  20  (fig.  7)  represents  an  oxbow  in  an  early  stage  of  forma- 
tion. Some  water  is  still  flowing  through  the  old  channel  ( egh ). 
The  cut-off  is  clearly  marked.  From  c to  d a sand  bar  is  being  con- 
structed and  now  almost  reaches  the  water  surface  and  extends 
from  shore  to  shore.  The  outlet  end  of  the  oxbow  will,  of  course, 
be  the  first  to  close.  Then  will  follow  the  filling  of  the  inlet,  thus 
completing  the  formation  of  a closed  body  of  water  having  the 


5°4 


BOTANICAL  GAZETTE 


[JUNE 


shape  of  a bow.  The  shores  of  the  newly  formed  lake  have  steep, 
vertical  sides  on  the  outside  of  the  stream  curve.  On  the  inside 


Fig.  7. — Map  of  oxbow  no.  20:  an  early  stage  in  the  development  of  an  oxbow 
lake;  main  stream  occupies  channel  abed;  some  water  still  flows  through  old  channel 
(egh);  from  c to  d a sand  bar  is  being  deposited;  shore  no.  I,  composed  of  sand,  gravel, 
and  boulders;  at  high  water  the  strip  may  be  covered;  shore  no.  II,  composed  of 
gravel,  sand,  and  fine  silt;  the  bare  soil  is  being  invaded  by  a mixed  association; 
shore  no.  Ill,  a mud  and  sand  flat  under  water  a good  part  of  the  year;  shore  no.  IV, 
cobblestone  on  which  no  vegetation  has  secured  foothold;  shores  nos.  V and  VI, 
perpendicular  erosion  shores;  “runs”  of  the  kind  shown  at  no.  VII  are  common 
throughout  the  Park,  being  very  narrow  and  with  vertical  walls. 


ROBBINS— BO  ULDER  PA  RK 


505 


1918] 


of  the  curves  there  are  gravelly,  sandy,  or  muddy  shores  of  deposi- 
tion. The  future  history  of  this  lake  is  now  largely  concerned  with 
the  activities  of  plants.  In  the  developing  of  this  oxbow  lake 
Callitriche  palustris  L.  and  Batrachium  trichophyllum  (Chaix.) 
Bossch.  are  the  first  representatives  of  pond  life.  The  main  stream 
throughout  the  Park  is  too  swift  to  allow  the  growth  of  any  vegeta- 
tion within  it  except  algae,  chiefly  Draparnaldia  acuta  (Agardh) 
Kuetz.  and  Prasiola  mexicana  J.  G.  Agardh.  These  are  attached 
to  the  rocks  in  the  stream  bed.  They  flourish  only  in  swift  running 
water.  Batrachium  trichophyllum  and  Callitriche  have  appeared 
in  the  still  water  back  of  the  sand  bar.  Encroachment  by  the 
vegetation  now  starts  in  from  all  sides,  and  the  area  quickly  comes 
to  willow  thicket.  Car  ex  utriculata  sometimes  becomes  inter- 
polated at  the  margin  between  willow  thicket  and  open  water. 
It  is  an  important  invader  of  these  shallow  oxbows.  It  is  frequently 
succeeded  by  Carex  variabilis , which  is  in  turn  followed  by  willow 
thicket.  An  oxbow  that  has  had  its  connection  with  the  main 
current  severed  usually  passes  through  the  same  stages  of  succession 
found  about  lake  shores. 

Oxbow  3 is  a somewhat  different  type  from  the  preceding.  The 
lake  is  shallow  and  has  a muddy  bottom,  over  which  numerous 
small  rocks  are  scattered.  The  orignal  rock  and  gravel  stream  bed 
is  thus  still  visible.  The  lake  is  free  from  water  during  the  latter 
part  of  the  season.  Alopecurus  fulvus  is  dominant  on  the  muddy 
bottom.  Associates  are  Ranunculus  reptans , Eleocharis  acicularis , 
and  Sparganium  angustij olium  Michx.  Carex  utriculata  is  rapidly 
invading  the  Alopecurus  society.  Mixed  with  Carex  is  Glyceria 
americana , Alopecurus  fulvus , and  a great  deal  of  moss.  Moss  often 
invades  the  Carex  utriculata  association,  preparing  a substratum 
upon  which  willow  thicket  may  build  more  readily. 

Oxbow  5 is  a small  and  shallow  pond  which  undergoes  periodic 
drying.  There  is  a well  developed  Eleocharis-Ranunculus  asso- 
ciation on  the  sandy  bottom.  It  is  being  invaded  by  Carex  utri- 
culata, after  which  comes  willow  thicket. 

Oxbow  6 exhibits  a pond  that  is  now  almost  filled  with  vegeta- 
tion. A very  small  area  of  open  water  still  remains.  The  lake  was 
narrow  and  filled  in  a manner  normal  to  gravelly  or  muddy  shores, 


BOTANICAL  GAZETTE 


[JUNE 


5°6 

that  is,  by  the  invasion  of  willow  thicket.  There  is  no  vestige, 
however,  of  the  first  associations  of  these  shores.  Carex  utriculata 
occupies  the  wettest  part  of  the  area.  It  is  followed  on  all  sides  by 
the  springy  sedge  moor  of  Hypnum , Carex  variabilis,  and  C. 
canescens  L.  Here  Carex  variabilis  is  building  upon  the  moss. 
The  principal  shrub  succeeding  C.  variabilis  is  Salix  chlorophylla. 

Oxbow  lakes  8 and  9 differ  in  a marked  degree  from  all  others 
in  the  Park.  It  will  be  seen  that  they  are  the  only  lakes  of  oxbow 
origin  that  occur  on  the  middle  terrace.  Obviously  they  are 
physiographically  much  older  than  those  of  the  lower  stream  terrace. 
They  are  very  shallow  and  annually  dry  up.  In  spite  of  their  age, 
they  have  not  filled  to  any  extent.  Rock  and  coarse  gravel,  with 
but  comparatively  little  finer  material  between,  make  up  the  pond 
bottom.  This  lack  of  plant  debris  is  associated  with  periodic 
drying,  and  the  exposure  of  the  area  to  the  winds.  Late  in  a 
particularly  dry  season,  the  level  bottom  becomes  dry  and  the 
strong  winds  blow  away  the  material  that  accumulates.  The 
chief  associations  over  almost  the  entire  lake  bottom  is  an  open  one 
of  Eleocharis  palustris  and  Ranunculus  reptans  (fig.  6).  Glyceria 
borealis  and  G.  grandis  are  rare  associates.  About  the  shore  edge 
Carex  utriculata  is  slowly  working  inward.  Carex  variabilis  or 
meadow  scrub  may  come  to  the  water’s  edge.  A few  clumps  of 
Salix  chlorophylla  and  S.  padophylla  at  the  edge  are  relicts  of  the 
old  stream  bank  stage.  Such  individuals  have  numerous  dead  and 
dying  branches. 

East  Lake  (figs.  8,  9)  is  an  old  oxbow  of  South  Boulder  Creek, 
from  which  it  is  now  separated  by  about  250  ft.  The  intervening 
area  is  a sedge  moor  alternating  with  willow  thicket.  Through  this 
the  lake  outlet  feebly  flows.  The  old  shore  line  of  the  creek  is 
distinct.  Mertensia  ciliata  (Torr.)  Don.  and  Senecio  triangularis 
Hook.,  typical  streamside  plants  in  the  region,  may  be  found 
sparingly  in  the  willow  thicket  of  the  lake.  Salix  chlorophylla  and 
S.  padophylla , with  the  two  associated  herbaceous  species,  are  relicts 
of  a streamside  flora.  There  is  further  evidence  of  the  oxbow  origin 
of  East  Lake.  The  stream  cut  into  the  terminal  moraine  and  made 
a strong  curve  toward  the  southwest,  working  into  its  bank  almost 
at  right  angles.  As  is  happening  at  many  places  in  the  present 


igi8]  ROBBINS— BOULDER  PARK  507 

stream  course,  the  shores  were  eroded  and  an  inlet  of  considerable 
width  and  depth  formed.  We  take  it  that  the  position  of  this  is 
represented  by  the  long  tongue  of  sedge  moor  that  extends  from 
the  lake  edge  to  the  meadow.  Soil  borings  here  show  deep  deposits 
of  peat  extending  to  the  meadow. 

At  present,  East  Lake  has  a flat  bottom  and  a uniform  depth  of 
about  1 ft.  The  bottom  is  of  mud.  The  lake’s  development  is 


Fig.  8. — View  of  East  Lake,  of  oxbow  origin;  beyond  is  portion  of  large  terminal 
moraine. 


natural  and  typical  of  an  oxbow  belonging  to  the  lower  terrace. 
The  associations  in  and  about  the  pond  are  arranged  concentrically, 
particularly  along  the  west  and  south  sides.  A small  amount  of 
Batrachium  and  Callitriche  is  found  in  the  water.  Car  ex  utriculata 
makes  a pure  association  chiefly  along  the  west  shore,  where  it  is 
rapidly  pushing  out  into  the  water.  The  plant  is  1-2  ft.  high  and 
spreads  by  means  of  creeping  rootstocks.  Its  typical  habitat  is 
still  water  not  exceeding  1 ft.  in  depth.  The  amount  of  plant 
remains  annually  deposited  by  it  is  considerable.  Furthermore,  it 


5°8 


BOTANICAL  GAZETTE 


[JUNE 


breaks  wave  action  and  thereby  facilitates  the  accumulation  of  sedi- 
ment between  its  closely  crowded  erect  stalks.  It  is  followed  by 
the  typical  sedge  moor,  and  this  by  willow  thicket  or  meadow. 


Fig.  9. — Map  of  East  Lake,  showing  surrounding  plant  associations 


Carex  utriculata,  however,  is  not  the  only  agent  in  the  invasion 
of  the  open  water.  Fig.  10  shows  a section  of  the  shore  edge  and 
sedge  moor.  It  will  be  noted  that  there  is  a distinct  elevated  rim 
at  the  water’s  edge.  This  elevated  rim  is  present  almost  entirely 
about  the  lake.  Such  a rim  is  commonly  found  along  the  streams, 
and  is  the  result  of  stream  cutting.  Its  occurrence  about  a lake 
may  be  taken  as  an  evidence  of  its  oxbow  origin;  not  conclusively, 


ROBBINS— BOULDER  PARK 


509 


1918] 

however,  for  the  rim  is  also  a feature  of  glacial  lakes  such  as  Red- 
rock  (20)  and  Park,  which  are  not  genetically  related  to  stream 
topography.  In  these  it  is  no  doubt  formed  by  the  sapping  action 
of  ice.  Of  course,  stream  action  and  ice  action  may  be  cooperative 
factors.  At  any  rate,  whatever  its  origin,  once  established  the 
rim  is  maintained  by  vegetative  building.  At  East  Lake  the  rim 
about  the  edge  has  a mean  height  of  about  16  inches.  In  texture 
it  is  a loose  and  spongy  mass  and  consists  of  living  and  decayed 


water  has  loose  and  spongy  texture  and  consists  of  living  and  decayed  plant  material, 
largely  masses. 

plant  material,  largely  mosses.  Just  back  of  the  rim  the  soil  is 
wetter,  more  compact,  and  the  character  of  the  vegetation  some- 
what different.  Being  actually  drier  than  the  moor  a foot  or  so 
landward,  it  supports  an  assemblage  of  plants,  many  of  which 
would  scarcely  be  expected  to  grow  at  the  water’s  edge.  Salix 
chlorophylla  and  Dasiophora  fruticosa  are  the  important  shrubs 
of  the  rim.  The  seeds  lodge  and  germinate  on  the  bare,  more  or 
less  perpendicular,  wall  of  the  rim,  and  the  young  plants  curve  out- 
ward and  upward  over  the  water.  Other  characteristic  rim  plants 
are  Betula  glandulosa,  Sedum  rhodanthum , Dodecatheon  radicatum, 
and  Poa  leptocoma  Bong.  The  importance  of  moss  in  the  building 
of  the  rim  should  be  noted.  As  the  rim  slowly  builds  out  over  the 


BOTANICAL  GAZETTE 


[JUNE 


510 

water,  it  sinks  by  its  own  weight,  thus  forming  the  flatter,  lower,  and 
more  compact  part  just  back  from  the  edge.  Willow  thicket  is 
invading  the  moor  in  many  places.  It  is  replaced  by  meadow  scrub. 
At  several  points  sedge  moor  passes  directly  into  sedge  moor,  thus 
omitting  the  thicket  and  scrub  stages  of  succession. 

The  principal  species  in  the  sedge  moor  of  Boulder  Park  is 
Carex  variabilis.  It  is  a peat  forming  species.  In  reaction,  the 
soil  of  the  sedge  moor  is  very  slightly  acid.  The  plants  of  the  asso- 
ciation stand  close  together.  There  is  always  an  abundance  of 
moss,  which  is  of  great  importance  in  the  building  of  peat.  The 
sedge  moor  becomes  marshy  during  the  spring  and  early  summer 
and  after  heavy  rains.  Then,  the  water  aids  in  the  packing  down 
of  dead  sedge  plants.  The  water  table  is  always  high  and  the  soil 
water  content  high  throughout  the  year.  Stratification  occurs 
to  some  extent.  The  following  species  form  a ground  layer: 
Androsace  subumbellata  (A.  Nels.)  Small,  Galium  trifidum  L., 
Crunocallis  chamissonis  (Esch.)  Greene,  Veronica  serpyllijolia  L., 
Alsine  longifolia  (Muhl.)  Brit.,  moss,  and  liverworts.  Caltha 
rotundifolia  (Huth.)  Greene  is  an  important  component  of  the  sedge 
moor.  It  is  not  a shade  plant  and  hence  does  not  do  as  well  in  the 
denser  parts  of  the  association  as  in  more  open  spots. 

Petasites  sagittata  Gray  is  prevernal  in  the  sedge  moor.  C allha 
is  the  characteristic  species  of  the  spring  aspect  (May  15- July  1). 
The  summer  aspect  (July  i-August  15)  is  marked  by  a large  num- 
ber of  sedges,  grasses,  and  other  herbs,  most  important  of  which  are 
Carex  variabilis,  Deschampsia  caespitosa  (L.)  Beauv.,  Hierochloa 
odorata  (L.)  R.  and  S.,  Sedum  rhodanthum  Gray,  Pedicularis  groen- 
landica  Retz.,  and  Agrostis  hiemalis  (Walt.)  B.S.P.  The  appear- 
ance of  gentians  the  latter  part  of  August  ushers  in  the  autumn 
aspect  (August  15-October  1).  Chief  of  these  are  Pleurogyne 
fontana  A.  Nels.  and  Gentiana  plebeya  Cham.  During  the  winter 
the  sedge  moor  is  a level  expanse  of  withered  shoots  and  leaves, 
chiefly  Carex. 

As  compared  with  drier  associations,  the  seasonal  aspects  of 
the  sedge  moor  change  slowly.  The  reason  for  this  is  partly  the 
fact  that  Carex  hides  other  forms  growing  within  it,  and  furthermore 
to  the  actual  paucity  of  species  in  this  area  as  compared  with  drier 


ROBBINS— BOULDER  PARK 


1918] 


5ii 


situations.  More  important,  however,  are  soil,  temperature,  and 
moisture  conditions.  The  maximum  seasonal  variations  of  soil 
temperature  in  typical  associations  during  the  summer  of  1909  (13), 
as  seen  in  the  following  table,  will  give  some  explanation  of  the 
rapidity  of  change  in  the  seasonal  aspects. 

Sedge  moor 70  F. 

Willow  thicket 8° 

Meadow  scrub 160 

Herbaceous  meadow 20° 

Dry  grassland 240 

It  will  be  noted  from  this  that  the  sedge  moor  has  the  least 
variable  soil  temperature  throughout  the  growing  season.  This 
condition  is  due  for  the  most  part  to  the  amount  and  texture  of 
the  vegetative  cover,  and  also  is  intimately  related  to  the  soil  water 
content.  Sedge  moor  vegetation  screens  the  soil  efficiently.  More- 
over, as  a result  of  its  high  water  content  the  specific  heat  of  sedge 
moor  soil  is  high  and  its  conductivity  of  heat  low.  In  the  dry 
grassland,  on  the  other  hand,  there  is  an  absence  of  a dense  vegeta- 
tive covering.  Here  the  soil  has  a low  specific  heat,  due  to  its 
dryness,  and  its  heat  conductivity  is  great.  • Dry  grassland  heats 
up  more  rapidly  and  cools  off  more  readily  and  to  a greater  depth 
sooner  than  either  meadow  or  sedge  moor.  As  regards  soil  tempera- 
ture and  soil  moisture,  the  sedge  moor  shows  less  seasonal  variation 
than  either  meadow  or  dry  grassland.  This  condition  appears  to 
be  correlated  with  the  lack  of  marked  seasonal  aspects.  Edaphic 
conditions  within  a community  control  the  seasonal  changes  of  the 
vegetative  covering. 

We  have  described  the  stages  in  the  development  of  the  flood 
plains  of  a mountain  park.  The  oxbows  and  oxbow  ponds  are 
prominent  features  of  these  flood  plains.  Boulder  and  gravel 
shores  or  sand  and  fine  silt  shores  are  the  initial  habitats.  They 
culminate  in  a temporary  meadow  climax.  Two  exceptionally 
distinct  ages  of  flood  plains  are  represented:  a recently  formed  one 
now  in  the  willow  thicket  stage,  and  an  older  one  on  the  middle 
terrace  in  the  meadow  or  temporary  climax  stage.  A consideration 
of  the  fate  of  the  meadow  will  be  discussed  later,  after  treatment  of 
the  glacial  lake  and  dry  grassland  series  of  succession. 


512 


BOTANICAL  GAZETTE 


[JUNE 


GLACIAL  LAKE  SUCCESSION 

The  origin  of  Park  and  Filled  Lakes,  the  only  glacial  basins  in 
Boulder  Park,  has  been  discussed.  It  was  pointed  out  that  these 
two  bodies  occupy  the  positions  of  two  ice  cores  that  were  left  on 
the  valley  floor  on  the  retreat  of  the  first  ice  mass.  Immediately 
following  the  melting  of  these  ice  cores  there  was  left  a cold  water 
lake  with  bare  gravel  and  stony  shores.  Park  Lake  was  formerly 
of  much  greater  size;  the  limits  of  the  old  shore  may  be  clearly 
seen  on  the  west  side  of  the  present  body  of  water.  This  filled  area 
is  now  in  the  sedge  moor  stage  of  development,  as  is  also  Filled 
Lake.  We  may ‘gain  some  idea  of  the  early  stages  in  the  develop- 
ment of  the  shore  vegetation  by  studying  the  alpine  lakes  which 
are  found  in  abundance  6 or  8 miles  west  of  Boulder  Park.  How- 
ever, this  difference  exists:  alpine  lakes  are  comparatively  well 
protected  by  cirque  walls  from  wind  effects,  whereas  those  of 
Boulder  Park  are  in  the  open.  It  is  believed  that  this  difference 
explains  the  tardy  development  of  forest  growth  in  the  Park,  and 
the  maintenance  of  the  temporary  climax  grassland. 

In  the  lakes  and  ponds  of  Park  Lake,  algae  are  the  only  free 
floating  plant  life.  Besides  the  numerous  microscopic  algae  which 
constitute  a portion  of  the  plankton  of  these  waters,  Mougeotia 
laetevirens  (A.  Braun)  Wittr.  and  Spirogyra  Weberi  Keutz.  make 
up  large  floating  masses  along  shores  undisturbed  by  waves. 
Anabaena  fl os-aquae  (Lyngbye)  Breb.  becomes  conspicuous  in  late 
July  when  it  appears  as  “ water  bloom”  over  the  entire  surface  of 
the  lake. 

Sparganium  angustij olium  Michx.  (fig.  ii)  forms  a well  defined 
aquatic  community  along  the  shore  edge  out  to  a depth  of  about 
2 ft.  It  is  of  much  importance  in  the  aquatic  successions.  It 
grows  equally  well  on  a mud  or  gravelly  bottom.  A dwarf  form 
occurs  at  the  south  of  Park  Lake  on  a low,  flat  area  over  which 
the  water  level  fluctuates.  Here  the  plants  grow  but  a few 
inches  high,  and  possess  short,  rather  thick,  leaves.  Such  plants 
mature  fruit  normally.  Callitriche  palustris  L.  is  found  in  shallow 
water.  It  is  most  abundant  where  protected  from  wave  action. 
Batrachium  trichophyllum  (Chaix)  Bossch.  is  one  of  the  first  aquatics 
to  secure  a foothold  in  the  ponds  of  the  Park.  It  is  the  most 


ROBBINS— BOULDER  PARK 


1918] 


513 


important  aquatic  in  the  filling  process.  Potamogeton  foliosus  Raf., 
P.  lonchites  Ruck.,  P.  alpinus  Balb.,  P.  interior  Rydb.,  P.  lucens  L., 
and  Myriophyllum  spicatum  L.  are  other  rather  rare  aquatics  to  be 
found  here. 

Aquatic  plants  play  an  important  part  in  the  life  history  of  the 
lake.  On  the  flat,  mud  shores  the  Sparganium  association  is 


Fig.  11. — West  shore  at  Park  Lake;  note  Sparganium  in  shallow  water,  and  zones 
of  sedge  moor  and  willow  thicket. 


immediately  succeeded  by  the  Eleocharis- Ranunculus  community. 
This  is  well  shown  at  the  southwest  shore  (fig.  1 2) . This  shore  is  flat 
and  gravelly  or  muddy,  with  a few  small  boulders  scattered  about. 
It  is  perennially  covered  with  an  inch  or  so  of  water  up  to  about 
July  1,  after  which  time  it  is  a mud  and  gravel  flat.  Dwarf  forms 
of  Sparganium  angustij olium  flourish  in  this  habitat.  Eleocharis 
acicularis  here,  as  elsewhere  in  such  habitats,  is  the  chief  invader 
of  the  bare  soil.  Following  close  behind  and  upon  pure  tufts  of 
Eleocharis  come  Allocarya  scopulorum  and  Alopecurus  fulvus,  in  the 


5T4 


BOTANICAL  GAZETTE 


[JUNE 


order  named.  Alopecurus  grows  in  caespitose  clumps  3-6  inches 
in  diameter.  These  tufts  are  the  nucleus  for  the  growth  of  such 
plants  as  Epilobium  Hornmannii  Reich.,  Agrostis  hiemalis , and 
Veronica  serpyllifolia  L.  Following  the  establishment  of  these 
herbs  come  Car  ex  variabilis,  C.  lanuginosa  Michx.,  C.f estiva  Dewey, 
Deschampsia  caespitosa,  Phleum  alpinum , and  Poa  leptocoma.  As 


Fig.  12. — View  of  Eleocharis-Ranunculus  association  at  southwest  corner  of  Park 
Lake;  community  occupies  a broad  mud  flat;  note  tufts  of  Alopecurus  fulvus. 

at  East  Lake,  sedge  moor  may  be  succeeded  directly  by  herbaceous 
meadow,  the  Car  ex  f estiva  society  being  the  first  meadow  community 
to  become  established. 

Filled  Lake  is  one  of  the  most  interesting  features  of  the  Park. 
This  old  lake  bed  is  now  in  the  sedge  moor  stage  of  development. 
The  shore  line  is  still  quite  distinct,  made  more  so  by  the  vegetative 
growth  than  by  any  topographic  condition.  Early  successions  now 
in  operation  at  various  places  in  the  Park  are  undoubtedly  similar 
to  those  which  led  up  to  the  present  sedge  moor  stage  in  Filled  Lake. 


ROBBINS— BOULDER  PARK 


5*5 


1918] 

The  association  immediately  preceding  the  present  one  and  the 
future  changes  are  quite  clear.  The  sedge  moor  is  now  dominated 
by  Carex  variabilis  and  C.  utriculata,  alternating  and  freely  mixing. 
Obviously,  C.  variabilis  has  followed  upon  C.  utriculata.  Mosses 


are  an  exceedingly  important  element  in  the  building  up  of  sedge 
moor. 

If  one  stands  at  the  old  shore  line  and  looks  out  over  the  flat, 
sedge-covered  lake  floor  it  is  seen  to  be  divided  into  two  quite  dis- 
tinct parts:  (1)  the  half  nearer  the  outlet  is  lighter  in  color,  due  to 
the  predominance  of  sedges,  of  which  C.  utriculata  forms  a good 
proportion;  (2)  the  north  half  is  darker,  due  to  a pronounced  admix- 
ture of  Deschampsia , and  a greater  proportion  of  C.  variabilis  as 
compared  with  C.  utriculata.  The  north  half  of  the  lake  is  drier. 


BOTANICAL  GAZETTE 


[JUNE 


516 


This  would  be  expected,  since  it  is  further  removed  from  the  outlet 
and  is  adjacent  to  the  hill  at  the  north,  from  which  it  has  received 
considerable  wash  material. 

Scattered  throughout  the  sedge  moor,  and  particularly  in  the 
upper  half  mentioned,  are  numerous  clumps  of  vegetation.  These 
are  slightly  elevated  above  the  general  level  and  vary  from  1 to  3 ft. 
in  diameter.  The  nucleus  of  a clump  is  usually  a Salix  chlorophylla 
shrub.  This  species  is  an  early  invader  of  sedge  moor  throughout 
the  Park.  Building  in  and  around  it  are  such  early  invaders  as 
Sedum  rhodanthum , Alsine  longifolia  (Muhl.)  Brit.,  Arabis  hirsuta 
Scop.,  Cerastium  occidentale  Greene,  Geum  strictum  Ait.,  and 
Dasiophora  fruticosa.  The  clumps  may  also  originate  about  a 


k 

Herbaceous-meadow  Meadow-scrub 


Fig.  14. — Ideal  section  at  west  shore  of  Park  Lake 


Dasiophora  shrub  or  Deschampsia  tuft.  All  the  shrubs  are  young. 
Ring  counts  of  Dasiophora  show  the  large  majority  to  be  13-15 
years  of  age.  Salix  shrubs  are  older  on  the  average.  The  sedge 
moor  is  rapidly  being  converted  into  meadow. 

At  the  northwest  shore  of  Filled  Lake  there  is  a shallow  shelf 
extending  outward  from  the  shore  line.  The  limits  of  this  shelf 
were  determined  by  making  soil  borings.  These  were  simply  used 
as  a check  on  the  determination  of  its  limits  by  the  vegetative 
covering.  In  fact,  the  presence  of  the  shelf  here  was  called  to  the 
attention  by  a rather  marked  difference  in  the  vegetation  as  con- 
trasted with  that  beyond.  It  is  mentioned  simply  to  illustrate 
transition  conditions  between  sedge  moor  and  meadow.  Here 
Deschampsia  caespitosa  is  predominant.  Young  Dasiophora  shrubs 
are  very  uniformly  distributed  throughout.  Sedges,  relatively,  are 
not  an  important  component.  Secondary  species  are:  Hierochloa 
odorata  (L.)  R.  and  S.,  Phleum  alpinum  L.,  Poa  leptocoma,  Cerastium 
occidentale , Alsine  longifolia , Caltha  rotundifolia  (Huth.)  Greene., 
Sedum  rhodanthum , Geum  strictum , Potentilla  gracilis , Valeriana 


i9i81 


ROBBINS— BOULDER  PARK 


517 


ceratophylla  (Hook.)  Piper,  Achillea  lanulosa  Nutt.,  Antennaria 
parvifolia  Nutt.,  and  Crepis  perplexans  Rydb.  The  large  number 
of  characteristic  meadow  species  will  be  noted. 

The  depth  of  the  peat  deposit  in  the  lake  was  determined 
throughout.  In  the  center  it  is  over  10  ft.  deep.  From  here  the 
depth  gradually  decreases  toward  the  shores.  The  rate  of  increase 
in  depth  may  be  judged  by  a set  of  borings  made  every  5 m.  along 
an  east-west  line  to  the  center  of  the  area.  Starting  at  the  east 
shore  this  series  shows  depth  (in  cm.)  as  follows:  40-46-47-43-70- 
136-180-2 1 2-2 20-2 58-over  300. 

Borings  show  that  the  lake  has  been  filled  almost  entirely  with 
the  stems  and  leaves  of  Carex.  The  surface  soil  is  loosely  packed 
plant  material,  readily  separated  into  layers,  indicating  seasonal 
deposition.  The  upper  6-8  inches  are  light  brown  in  color;  below 
this,  the  layers  become  darker  and  more  compact.  This  soil 
exhibits  a slight  acid  reaction. 

It  will  be  recalled  that,  at  East  Lake,  Salix  chlorophylla  is  a 
characteristic  plant  of  the  raised  rim  at  the  water’s  edge.  A similar 
condition  exists  here.  At  the  west  shore  there  is  a very  distinct 
line  of  this  shrub,  on  a more  or  less  evident  rim;  at  one  time  these 
formed  a fringe  at  the  water’s  edge.  Back  of  this  rim  is  a belt 
averaging  about  20  ft.  wide,  clearly  the  old  sedge  moor  of  the  lake 
shore.  Beyond  this  is  a meadow  scrub,  followed  by  herbaceous 
meadow,  then  dry  grassland.  The  dry  grassland  is  not  the  outcome 
of  progressive  drying  of  the  meadow,  as  the  zonation  might  sug- 
gest, but  it  represents  a stage  in  a xerarch  succession  on  the  glacial 
gravel  of  the  high  terrace. 

DRY  GRASSLAND  SUCCESSION 

The  rapid  drainage  of  the  lake  which  covered  the  entire  park 
left  a level,  uniformly  gravelly,  area  exposed  to  the  drying  and 
mechanical  effects  of  the  winds,  and  the  extremes  of  diurnal  and 
yearly  temperatures.  Lichens  and  Selaginella  densa  are  the  chief 
pioneers  of  the  glacial  gravels  here.  The  latter  is  a mat  former,  and 
on  the  mats  other  plants  gain  a foothold.  Its  reaction  upon  the 
habitat,  in  holding  the  soil,  adding  humus,  and  retaining  water, 
favors  the  entrance  of  such  xerophytes  as  Erigeron  multifidus  Rydb., 


5i8 


BOTANICAL  GAZETTE 


[JUNE 


Sedum  stenopetalum,  Potentilla  concinna  Rich.,  Car  ex  stenophylla 
Wahl.,  Aragallus  Lambertii  (Pursh)  Greene,  Chrysopsis  villosa 
(Pursh)  Nutt.,  Comandra  pallida  A. DC.,  Arenaria  Fendleri  Gray, 
Artemisia  frigida  Willd.,  and  A . canadensis  Michx. ; simultaneously, 
there  is  an  incoming  of  such  grasses  as  Muhlenbergia  gracilis  Trin., 
Danthonia  Parryi  Scribn.,  Festuca  saximontana  Rydb.,  Poa  interior 
Rydb.,  and  Koeleria  cristata  Pers.  There  results  a xerophytic 
grassland  which  has  been  designated  “dry  grassland.”  It  is  a 
persistent  and  long-lived  plant  community. 

The  dry  grassland  of  Boulder  Park  has  been  the  object  of 
extended  study  by  Ramaley  (14,  15,  16,  17).  The  association  is 
preeminently  one  of  coarse,  gravelly,  thin  soils.  Humus  is  con- 
spicuously scarce.  The  soil  temperatures  run  high  throughout 
the  vegetative  season,  and  the  soil  water  content  low,  at  times 
falling  below  the  wilting  coefficient.  The  area  is  well  exposed  to  the 
winds,  and  snow  does  not  accumulate  to  any  extent.  Ramaley 
has  shown  that  70  per  cent  of  the  most  important  dry  grassland 
plants  are  shallow  rooted,  and  that  33  per  cent  of  them  are  rhizo- 
matous;  moreover,  many  of  those  which  do  not  bear  rhizomes 
have  much  branched  caudices.  Practically  91  per  cent  of  the  dry 
grassland  plants  are  perennial.  These  facts  point  to  the  extreme 
xerophytism  of  the  habitat. 

The  dry  grassland  is  an  open  community;  bare  ground  composes 
about  15  per  cent  of  the  whole  area  during  the  month  of  July. 
There  is  ample  opportunity  for  seeds  to  find  open  territory;  but 
the  life  of  the  seedling  is  a precarious  one.  There  is  a lack  of  soil 
water,  droughts  are  frequent  in  summer,  the  transpiration  rate  is 
high,  and  there  is  a lack  of  winter  snow  cover.  These  conditions 
exclude  the  invasion  of  trees  and  many  mesophytic  plants. 

Dry  grassland  has  all  appearances  of  being  the  ultimate  vegeta- 
tion throughout  the  Park,  under  present  climatic  and  physiographic 
conditions  at  least.  However,  slowly  but  surely  it  is  being  invaded 
in  places  by  meadow;  a series  of  dry  years  may  see  the  drying  up 
of  meadow,  the  fragmentation  of  plant  parts,  and  their  removal 
by  wind,  thus  reinstating  the  dry  grassland  stage.  The  resultant 
is  a slow  encroachment  of  dry  grassland  by  mesophytic  grasses  and 
other  herbs.  As  has  been  indicated,  if  physiographic  and  climatic 


ROBBINS— BOULDER  PARK 


5*9 


1918] 


conditions  remain  unchanged,  the  process  of  encroachment  will  be 
extremely  slow.  However,  physiographic  agencies  are  destroying 
the  dry  grassland  habitat  at  a rate  which  exceeds  that  of  biotic 
agencies.  At  many  points  the  high  terrace  is  being  eroded  by  the 
stream,  and  invariably  the  flood  plain  temporarily  culminates  in 
meadow;  again,  debris  is  accumulating  at  the  bases  of  slopes  and 
in  the  depressions  between  glacial  hummocks.  On  this  fine  grained 
and  deeper  soil,  with  its  greater  water  retentiveness,  meadow 
species  become  well  established;  hence  it  is  seen  that  the  combined 
activities  of  biotic  and  physiographic  factors  are  resulting  in  the 
slow  disappearance  of  the  dry  grassland  and  the  establishment 
thereupon  of  a mesophytic  grassland.  Fuller  (6)  points  out  that 
whereas  the  hydrarch  succession  of  Boulder  Park  is  closely  com- 
parable to  that  of  the  Illinois  prairie,  the  Park  area  exhibits  a 
xerarch  succession  comparable  to  nothing  found  in  Illinois. 

Two  types  of  meadow  are  displayed  in  the  Park  which  we  have 
designated  “ herbaceous  meadow”  and  “ meadow  scrub.”  The 
latter  consistently  occupies  moister  situations,  and  very  frequently 
precedes  herbaceous  meadow  in  the  succession. 

The  seasonal  aspects  and  detailed  structure  of  the  meadow  need 
not  be  entered  into  extensively  here.  Reed  (21)  has  given  us  a 
report  of  the  chief  meadow  societies  in  the  Park,  together  with  a 
list  of  the  meadow  plants  with  their  frequency  and  soil  moisture 
index. 

Carex  festiva  forms  distinct  meadow  societies  on  the  middle 
terrace  and  about  the  lakes.  Where  the  slope  of  the  lake  shore  is 
gradual,  sedge  moor  immediately  joins  on  to  this  society.  Its 
chief  associates  are  Deschampsia  caespitosa,  Phleum  alpinum, 
Potentilla  gracilis  Dough,  Poa  Buckley  ana  Nash,  and  Poa  pratensis 
L.  A conspicuous  society  of  Pedicularis  Parryi  Gray  occurs  just 
outside  the  Carex  festiva  society  in  soil  that  is  drier  and  more  shal- 
low. A quadrat  census  of  the  plants  of  the  society  showed  the 
principal  species  to  be  Potentilla  gracilis , Astragalus  alpinus  L., 
Pseudocymopterus  sylvaticus  A.  Nels.,  and  Chondrophylla  Fremontii 
(Torr.)  A.  Nels.  The  most  unobservant  person  would  remark 
about  the  well  defined  limits  of  the  Pentstemon  procerus  Dough 
society.  It  extends  in  a semicircle  about  the  south  flank  of  a low, 


52° 


BOTANICAL  GAZETTE 


[JUNE 


morainal  elevation  on  the  east  side  of  Park  Lake,  and  is  also  char- 
acteristic of  glacial  sinks.  Grasses  do  not  form  a close  growth,  but 
other  herbaceous  species  predominate.  Troximon  glaucum  Nutt., 
Potentilla  gracilis , and  Valeriana  edulis  Nutt,  are  the  principal 
associates.  A society,  the  main  representatives  of  which  are 

DIAGRAM  OF  PLANT  SUCCESSION,  BOULDER  PARK,  COLORADO 


Erigeron  macranthus  Nutt.,  Campanula  Parryi  Gray,  and  Eriogonum 
subalpinum  Greene,  is  characteristic  of  the  meadow  that  imme- 
diately adjoins  dry  grassland.  Associated  species  are  Galium 
boreale  L.,  Achillaea  lanulosa  Nutt.,  Stipa  Nelsonii  Scribn.,  Poa 
interior  Rydb.,  and  Koeleria  cristata  (L.)  Pers. 

Discussion 

Based  upon  the  water  content  of  the  initial  habitat*  the  suc- 
cessions may  be  classified  as  hydrarch  and  xerarch.  The  hydrarch 


ROBBINS— BOULDER  PARK 


521 


1918] 


succession  involves  the  glacial  lakes,  and  the  flood  plains  with 
deposition  banks  of  silt  and  sand;  the  xerarch  succession  involves 
the  gravels  laid  bare  by  the  rapid  drainage  of  a glacial  lake  which 
at  one  time  occupied  the  greater  part  of  the  Park,  and  the  flood 
plains  with  shores  of  coarse  gravel  or  shingle.  The  stages  in  these 
series  lead  to  a temporary  meadow  climax  (subclimax).  The 
climax  is  reached  much  sooner  by  the  hydrarch  than  by  the  xerarch 
series.  In  fact,  much  of  the  high  terrace  is  now  in  the  dry  grassland 
stage,  which  under  existing  environmental  conditions,  incident 
to  the  topographical  setting  of  the  Park,  promises  to  be  long  stand- 
ing. However,  there  is  ample  evidence  that  even  under  these 
circumstances  it  is  being  slowly  replaced  by  meadow.  More- 
over, the  physiographic  operations  now  in  progress,  namely,  the 
erosion  of  Boulder,  Meadow,  and  Trestle  Creeks,  and  the 
accumulation  of  wash  material  at  the  bases  of  slopes,  look  toward 
the  disappearance  of  the  dry  grassland  habitat  and  the  ushering  in 
of  meadowland. 

The  question  now  arises,  are  trees  advancing  upon  the  meadow  ? 
The  forest  associations  bordering  the  Park  are  as  follows:  (1)  aspen 
(. Populus  tremuloides  Michx.),  (2)  lodgepole  pine,  (3)  (. Finns  Mur- 
rayana  Oreg.  Com.),  and  (3)  Engelmann  spruce- sub  alpine  fir 
(. Picea  Engelmannii  [Parry]  Engelm. — Abies  lasiocarpa  [Hook.] 
Nutt.).  Aspen  often  forms  a fringe  between  the  coniferous  asso- 
ciations and  the  meadow  of  the  open  Park.  This  relation  prevails 
throughout  the  Rocky  Mountain  region.  Wherever  the  develop- 
mental series  has  led  up  to  the  meadow  stage,  however,  as  it  has 
in  Boulder  Park,  this  stage  bids  fair  to  hold  its  ground  against  the 
invasion  of  trees,  thus  constituting  a climax  (subclimax)  of  long 
duration.  The  principal  factor  involved  here  is  competition.  It 
may  occur  to  one  that,  although  the  competition  of  meadow  species 
prevents  the  forestation  of  the  open  Park,  there  is  nothing  to  prevent 
the  dry  grassland  with  its  abundance  of  open  ground  being  invaded 
by  trees.  In  this  connection  it  may  be  said  that  the  exposure  of 
the  dry  grassland  to  excessive  evaporation  as  conditioned  by  wind, 
temperature,  and  the  lack  of  a snow  cover  makes  a situation  in 
which  trees  find  it  impossible  to  get  a start.  The  climatic  climax 
of  the  region  is  a forest  of  Englemann  spruce-subalpine  fir. 


522 


BOTANICAL  GAZETTE 


[JUNE 


In  comparing  the  mountain  lakes  with  those  of  the  plains  and 
lower  altitudes  generally,  it  is  striking  that  those  of  lower  elevations 
support  the  richer  aquatic  vegetation.  The  ponds  of  Boulder 
Park  do  not  have  many  species  commonly  known  as  belonging  to 
the  water  habitat.  For  example,  there  will  be  noted  the  total 
absence  of  species  of  Lenina,  Hydrocharis,  Ceratophyllum,  Utri- 
cularia,  Riccia,  Azolla,  and  Salvinia,  free  floating  species  common  at 
lower  altitudes.  Of  these,  several  Lemna  spp.  and  Vtricularia 
vulgaris  have  been  collected  at  elevations  in  Colorado  as  high  as 
Boulder  Park.  Many  submersed  and  emersed  fixed  species  are  not 
to  be  found  here.  Among  such  may  be  mentioned  species  of  Nitella , 
Isoetes,  Naias,  Elodea,  Nymphaea,  and  a number  of  Potamogeton 
spp.  Several  other  species  of  Potamogeton,  Nymphaea  polysepala 
(Engelm.)  Greene,  Isoetes  Bolanderi  Engelm.,  /.  paupercula 
(Engelm.)  A.  A.  Eaton,  and  Naias  gaudalupensis  (Spreng.)  Morong 
are  reported  from  a few  lower  altitudes  in  Colorado. 

The  scarcity  of  aquatic  plant  life  in  the  lakes  and  ponds  of  the 
Park  is  in  part  due  to  the  coldness  of  the  waters  during  a consider- 
able portion  of  the  year.  Moreover,  the  marsh  type  of  vegetation 
here  is  meager,  and  little  shelter  is  offered  to  many  free  floating 
forms.  The  lakes  and  ponds  in  Boulder  Park  contain  very  soft 
water.  No  doubt  the  same  is  true  of  most  high  altitude  lakes. 
Nearly  all  plains  lakes,  however,  are  rich  in  alkali  salts;  bicar- 
bonates of  calcium  and  magnesium,  also  of  potassium  and  sodium, 
are  quite  universally  present.  A number  of  workers  (2,  5,  25)  have 
noted  that  waters  rich  in  lime  carbonates  have  a richer  aquatic 
flora  and  fauna  than  soft  waters.  In  the  absence  of  free  carbon 
dioxide,  water  plants  may  make  use  of  the  half-bound  carbon 
dioxide  of  bicarbonates,  chiefly  those  of  calcium  and  magnesium, 
dissolved  in  the  water.  Undoubtedly  the  kind  and  quantity  of 
dissolved  salts  in  lake  waters  is  an  important  factor  in  controlling 
vegetative  development.  In  Boulder  Park  lakes  and  ponds  the 
absence  of  these  salts  is  quite  likely  a most  important  factor  limiting 
the  growth  of  algae  and  other  submerged  aquatics. 

The  total  absence  of  Scirpus,  Typha,  and  Phragmites  reed 
swamps  in  Boulder  Park  will  be  noted. 


ROBBINS— BOULDER  PARK 


523 


19 1 8] 

Sphagnum  bogs. — Sphagnum  moss  is  found  in  very  small 
amounts  here  and  there  in  the  Park,  but  in  no  place  is  there  any 
approach  to  the  formation  of  sphagnum  moor.  Small  sphagnum 
moors  are  occasionally  found  at  higher  elevations  in  heavily 
forested  areas  in  northern  Colorado,  but  never  are  they  as  well 
developed  and  characteristic  as  those  found  north  and  east  in  the 
United  States. 

For  the  optimum  development  of  sphagnum,  there  must  be 
abundant  precipitation,  slow  evaporation  from  the  surface,  slow 
percolation  and  run-off  of  soil  water,  low  temperature,  and  absence 
of  drying  winds.  In  only  favored  situations  are  such  conditions 
found  in  Colorado.  Boulder  Park  is  a very  unfavorable  locality 
for  the  development  of  sphagnum  moors.  Here  the  drainage  is 
generally  good,  the  temperatures  of  both  air  and  soil  may  run  high, 
at  least  for  a short  period,  many  seasons  are  dry,  and  the  winds  are 
desiccating.  In  his  description  of  the  bog  plant  societies  of  northern 
North  America,  Transeau  (24)  has  selected  15  characteristic  bog 
plants:  Menyanthes  trifoliata,  Dulichium  arundinaceum , Comarum 
palustre,  Scheuchzeria  palustris,  Eriophorum  polystachon , Drosera 
rotundifolia,  Sarracenia  purpurea , Oxy coccus  oxy coccus,  Chio genes 
hispidula , Andromeda  polijolia , Chamaedaphne  calyculata,  Ledum 
groenlandicum , Kalmia  glauca , Betula  pumila , and  Larix  laricina. 
Of  these,  Eriophorum  polystachyon  is  the  only  one  found  in  Boulder 
Park,  and  it  is  rare.  Menyanthes  trifoliata  has  been  found  in  a bog 
a number  of  miles  north  of  Boulder  Park. 

The  writer  is  indebted  to  Dr.  H.  C.  Cowles,  under  whom  this 
study  was  conducted,  for  his  helpful  suggestions  and  criticisms;  to 
Professor  Francis  Ramaley  for  valuable  advice  and  for  laboratory 
facilities  granted  at  the  Mountain  Laboratory  for  Field  Biology 
(Tolland,  Colorado);  to  Professor  Aven  Nelson,  who  identified 
a large  number  of  the  specimens;  and  to  Mr.  A.  S.  Hitchcock  for 
the  identification  of  some  difficult  species  of  Poa. 

State  Agricultural  College 
Fort  Collins,  Colo. 


524 


BOTANICAL  GAZETTE 


[JUNE 


LITERATURE  CITED 

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15:790-804.  1907. 

2.  Birge,  E.  A.,  and  Juday,  C.,  The  inland  lakes  of  Wisconsin;  dissolved 
gases  and  their  biological  significance.  Wis.  Surv.  Bull.  22.  1911. 

3.  Bruderlin,  Katherine,  A study  of  the  lodgepole  pine  forests  of  Boulder 
Park,  Tolland,  Colo.  Univ.  Colo.  Studies  8:265-275.  1911. 

4.  Capps,  S.  R.,  and  Leffingwell,  E.  D.  K.,  Pleistocene  geology  of  the 
Sawatch  Range  near  Leadville,  Colo.  Jour.  Geol.  12:698-706.  1904. 

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dioxide  with  speciall  reference  to  the  carbonates.  23d  Ann.  Rep.  Mo. 
Bot.  Gard.  171-207.  1912. 

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the  prairie  of  Illinois.  Trans.  III.  Acad.  Sci.  8:1-10.  1917. 

7.  Gwillim,  J.  C.,  Glaciation  in  the  Atlin  district,  British  Columbia.  Jour. 
Geol.  10:182-186.  1902. 

8.  Hershey,  O.  H.,  The  relation  between  certain  river  terraces  and  the 
glacial  series  in  Northwestern  California.  Jour.  Geol.  11:431-459.  1903. 

9.  Jefferson,  W.  D.,  A certain  type  of  lake  formation  in  the  Canadian 
Rocky  Mountains.  Jour.  Geol.  7:247-261.  1899. 

10.  Ramaley,  Francis,  Plant  zones  in  the  Rocky  Mountains  of  Colorado. 
Science  N.S.  26:642-643.  1907. 

11.  , Botanical  opportunity  in  Colorado.  Univ.  Colo.  Studies  6:5-10. 

1908. 

12.  , The  University  of  Colorado  Mountain  Laboratory.  Univ.  Colo. 

Studies  7:91-95.  1909. 

13.  , Northern  Colorado  plant  communities  with  special  reference  to 

Boulder  Park.  Univ.  Colo.  Studies  7:223-236.  1910. 

14.  , The  amount  of  bare  ground  in  some  mountain  grasslands.  Bot. 

Gaz.  57:526-528.  1914. 

15.  , The  relative  importance  of  different  species  in  a mountain  grass- 

land. Bot.  Gaz.  60:154-157.  1915. 

16.  , Quadrat  studies  in  a mountain  grassland.  Bot.  Gaz.  62:70-74. 

1916. 

17.  , Dry  grassland  of  a high  mountain  park  in  Northern  Colorado. 

Plant  World  19: 249-270.  1916. 

18.  Ramaley,  Francis,  and  Mitchell,  L.  A.,  Ecological  cross-section  of 
Boulder  Park  (Tolland,  Colo.).  Univ.  Colo.  Studies  8:277-287.  1911. 

19.  Ramaley,  Francis,  and  Robbins,  W.  W.,  A summer  laboratory  for  moun- 
tain botany.  Plant  World  12: 105-1 10.  1909. 

20.  , Studies  in  lake  and  streamside  vegetation.  I.  Redrock  Lake 

near  Ward,  Colo.  Univ.  Colo.  Studies  6:1.  1909. 


ROBBINS— BOULDER  PARK 


525 


1918] 

21.  Reed,  E.  L.,  Meadow  vegetation  in  the  montane  region  of  Northern 
Colorado.  Bull.  Torr.  Bot.  Club  44:97-109.  1917. 

22.  Robbins,  W.  W.,  Native  vegetation  and  climate  of  Colorado  in  their  rela- 
tion to  agriculture.  Colo.  Agric.  Exper.  Sta.  Bull.  224:1-56.  1917. 

23.  Salisbury,  R.  D.,  and  Blackwelder,  E.,  Glaciation  in  the  Bighorn 
Mountains.  Jour.  Geol.  11:216-223.  1903. 

24.  Transeau,  E.  N.,  On  the  geographical  distribution  and  ecological  relations 
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25.  Wesenberg-Lund,  Comparison  of  Scottish  with  Danish  lakes.  Revue 
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26.  Westgate,  L.  H.,  The  Twin  Lakes  glaciated  area,  Colorado.  Jour.  Geol. 
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